CVE-2025-21897 is a vulnerability identified in the Linux kernel's scheduler extension (sched_ext) subsystem, specifically within the pick_task_scx() function. The scheduler is responsible for managing task execution on CPUs, ensuring efficient and fair CPU time allocation. The vulnerability arises from a logic flaw in how pick_task_scx() selects tasks when it is invoked without a preceding call to balance_scx(), a function that normally balances run queues. The root cause is that pick_task_scx() may return a task that is not actually runnable, due to incorrect checks on the task's state. The workaround implemented previously incorrectly relied on whether the previous task (@prev) was on the scheduling context (SCX) to decide if the task should continue running. However, a task can be on SCX but not runnable, leading to pick_task_scx() returning a non-runnable task. This can cause the system to enter inconsistent states such as a task being marked as not on the run queue (!on_rq) but still on CPU (on_cpu), which is logically contradictory. Such states can cause potential wakers to busy loop, leading to CPU resource exhaustion and possible deadlocks. The fix involves refining the check to ensure that the task is actually queued (SCX_TASK_QUEUED) before being selected, thus preventing non-runnable tasks from being scheduled. This correction improves scheduler stability and prevents subtle deadlock or livelock conditions caused by the previous flawed logic. No known exploits are currently reported in the wild, and the vulnerability affects specific Linux kernel versions identified by the commit hash a6250aa251eacaf3ebfcfe152a96a727fd483ecd. The issue is technical and low-level, impacting kernel task scheduling behavior rather than directly exposing memory corruption or privilege escalation vectors.

For European organizations relying on Linux-based systems, especially those running custom or specific kernel versions affected by this flaw, the vulnerability poses risks primarily to system stability and availability. Servers, cloud infrastructure, and embedded systems using the affected Linux kernel versions may experience scheduler deadlocks or livelocks, leading to CPU resource starvation and potential system hangs or crashes. This can disrupt critical services, degrade performance, and increase downtime. While the vulnerability does not directly lead to confidentiality or integrity breaches, the availability impact can be significant for high-availability environments such as financial institutions, telecommunications providers, and critical infrastructure operators common in Europe. Systems running containerized workloads or real-time applications may be particularly sensitive to scheduler anomalies. Since no known exploits exist yet, the immediate risk is moderate, but unpatched systems remain vulnerable to stability issues that could be triggered under specific workload conditions. The complexity of the issue means that detection may be difficult without detailed kernel tracing or monitoring, increasing the risk of unnoticed degradation in system performance.

European organizations should prioritize updating their Linux kernels to versions that include the fix for CVE-2025-21897. This involves applying the patch that correctly checks the SCX_TASK_QUEUED flag before scheduling tasks. Kernel maintainers and distributors should ensure that updated kernel packages are promptly released and deployed. For organizations managing their own kernel builds, integrating the patch from the commit a6250aa251ea is essential. Additionally, system administrators should implement enhanced monitoring of CPU scheduling behavior and system load to detect anomalies such as unexpected CPU busy loops or deadlocks. Employing kernel tracing tools (e.g., ftrace, perf) can help identify symptoms related to this vulnerability. In environments where immediate patching is not feasible, workload scheduling policies can be adjusted to reduce the likelihood of triggering the scheduler bug, such as limiting highly concurrent or CPU-intensive tasks that stress the scheduler. Finally, maintaining robust system backups and failover mechanisms will mitigate the impact of potential system hangs or crashes caused by this issue.